zbackup enhancements: set/unset/list; error reporting; restructured documentation

These changes only affect zbackup, not the underlying zsnap and zreplicate.  I also restructured the documentation so that most of the description for zbackup is in a separate README, as I found I was cluttering up the top-level README with too much detail.

The set/unset/list enhancement provides a comfortable interface for setting and querying the ZFS properties which drive the zbackup behaviour.

cheers,
Simon